Chromatography: Monosaccharides (AQA A Level Biology)

• Chromatography is a technique that can be used to separate a mixture into its individual components
• Chromatography relies on differences in the solubility of the different chemicals (called ‘solutes’) within a mixture
• All chromatography techniques use two phases:
  • The mobile phase
  • The stationary phase

• The components in the mixture separate as the mobile phase travels over the stationary phase
• Differences in the solubility of each component in the mobile phase which affects how far each component can travel
• Those components with higher solubility will travel further than the others
• This is because they spend more time in the mobile phase and are thus carried further up the paper than the less soluble components

Paper Chromatography

• Paper chromatography is one specific form of chromatography
• In paper chromatography:
  • The mobile phase is the solvent in which the sample molecules can move, which in paper chromatography is a liquid e.g. water or ethanol
  • The stationary phase in paper chromatography is the chromatography paper

Paper chromatography method

• A spot of the mixture (that you want to separate) is placed on chromatography paper and left to dry
• The chromatography paper is then suspended in a solvent
• As the solvent travels up through the chromatography paper, the different components within the mixture begin to move up the paper at different speeds
  • Larger molecules move slower than smaller ones
  • This causes the original mixture to separate out into different spots or bands on the chromatography paper

• This produces what is known as a chromatogram

An example of a chromatogram that has been produced by using paper chromatography to separate a spot of ink

Using chromatography to separate a mixture of Monosaccharides

• Paper chromatography can be used to separate a mixture of monosaccharides
• Mixtures containing coloured molecules, such as ink or chlorophyll, do not have to be stained as they are already coloured
• Mixtures of colourless molecules, such as a mixture of monosaccharides, have to be stained first
• A spot of the stained monosaccharide sample mixture is placed on a line at the bottom of the chromatography paper
• Spots of known standard solutions of different monosaccharides are then placed on the line beside the sample spot
• The chromatography paper is then suspended in a solvent
• As the solvent travels up through the chromatography paper, the different monosaccharides within the mixture separate out at different distances from the line
• The unknown monosaccharides can then be identified by comparing and matching them with the chromatograms of the known standard solutions of different monosaccharides
  • If a spot from the monosaccharide sample mixture is at the same distance from the line as a spot from one of the known standard solutions, then the mixture must contain this monosaccharide

How chromatography can be used to separate a mixture of monosaccharides and identify the individual components